147 



Abstract— The effects of seasonal and 

 regional differences in diet composition 

 on the food requirements of Steller sea 

 lions (Eumetopias jubatus) were esti- 

 mated by using a bioenergetic model. 

 The model considered differences in 

 the energy density of the prey, and dif- 

 ferences in digestive efficiency and the 

 heat increment of feeding of different 

 diets. The model predicted that Steller 

 sea lions in southeast Alaska required 

 45-60% more food per day in early 

 spring (March) than after the breed- 

 ing season in late summer (August) 

 because of seasonal changes in the 

 energy density of the diets (along with 

 seasonal changes in energy require- 

 ments). The southeast Alaska popula- 

 tion, at 23,000 (±1660 SD) animals (all 

 ages), consumed an estimated 140,000 

 (±27,800) t of prey in 1998. In contrast, 

 we estimated that the 51,000 (±3680) 

 animals making up the western Alaska 

 population in the Gulf of Alaska and 

 Aleutian Islands consumed just over 

 twice this amount (303,000 [±57,500] 

 t). In terms of biomass removed in 1998 

 from Alaskan waters, we estimated 

 that Steller sea lions accounted for 

 about 5'^/c of the natural mortality of 

 gadids (pollock and cod) and up to 75% 

 of the natural mortality of hexagram- 

 mids (adult Atka mackerel). These two 

 groups of species were consumed in 

 higher amounts than any other The 

 predicted average daily food require- 

 ment per individual ranged from 16 

 l±2.8) to 20 (±3.6) kg (all ages com- 

 bined). Per capita food requirements 

 differed by as much as 24% between 

 regions of Alaska depending on the rel- 

 ative amounts of low-energy-density 

 prey (e.g. gadids) versus high-energy- 

 density prey (e.g. forage fish and 

 salmon) consumed. Estimated require- 

 ments were highest in regions where 

 Steller sea lions consumed higher 

 proportions of low-energy-density prey 

 and experienced the highest rates of 

 population decline. 



Prey consumption of Steller sea lions 

 {Eumetopias jubatus) off Alaska: 

 How much prey do they require? 



Arliss J. Winship 



Andrew W. Trites 



Department of Zoology and Marine Mammal Research Unit 



Fisheries Center. Room 18, Hut B-3 



University of British Columbia 



6248 Biological Sciences Road 



Vancouver, Bntish Columbia, Canada, \/6T 1Z4 



E-mail address (for A J. Winship) winship@zoology-Ubc.ca 



Manuscript accepted 20 September 2002. 

 Fish. Bull. 101:147-167 (2003). 



Nutritional stress may account for the 

 decline of Steller sea lions {Eumetopias 

 jubatus) in Alaska (Alverson, 1992), 

 which have declined by over 70% in the 

 last 20-25 years (Loughlin et al, 1992; 

 Trites and Larkin, 1996). Merrick et 

 al. (1997) found a negative correlation 

 between Steller sea lion diet diversity 

 and the rate of population change among 

 six regions of Alaska in the early 1990s. 

 The greatest rates of population decline 

 occurred in areas with low diet diversity, 

 where Steller sea lions predominantly 

 preyed on either walleye pollock or Atka 

 mackerel. Steller sea lions from areas 

 that did not experience a decline, or expe- 

 rienced a lower rate of decline, preyed on 

 both walleye pollock and Atka mackerel 

 along with several other groups of prey 

 species. 



Merrick et al. (1997) suggested that 

 the relationship between diet diver- 

 sity and the rate of population decline 

 reflected differences in the efficiency 

 with which Steller sea lions could find, 

 capture, and handle different numbers 

 of prey categories. However, the en- 

 ergy content of the diet may also have 

 a substantial effect on the foraging ef- 

 ficiency of Steller sea lions. Steller sea 

 lions consuming a low diversity diet of 

 primarily low-energy-density species 

 (e.g. gadids) need to consume more prey 

 biomass than Steller sea lions eating a 

 more diverse diet including high-en- 

 ergy-density species (e.g. forage fish, 

 salmon) to obtain the same amount of 

 energy. Thus, it may be more difficult 

 for Steller sea lions consuming a diet of 

 low-energy content to meet their energy 

 requirements or to forage efficiently 



than it would be for animals consuming 

 prey of high-energy content. 



The overall goal of our study was to 

 estimate the amount of prey required 

 by Steller sea lions in Alaska during 

 the 1990s using a previously devel- 

 oped bioenergetic model (Winship et 

 al., 2002). Our first objective was to 

 examine how daily food biomass re- 

 quirements were affected by seasonal 

 differences in the energy density of the 

 diet of Steller sea lions in southeast 

 Alaska. Our second objective was to 

 use the same model to compare the 

 food requirements of Steller sea lions 

 among seven regions of Alaska during 

 the 1990s (regions based on Merrick et 

 al., 1997, and Sease and Loughlin, 1999; 

 Fig. 1). Our third objective was to use 

 data from 1998 to compare estimates 

 of Steller sea lion prey consumption 

 with fisheries catches and estimates of 

 prey stock sizes (and natural mortality 

 rates). 



Methods 



Model structure 



The bioenergetic model that we used 

 is described in detail by Winship et al, 

 (2002). In brief, gross energy require- 

 ments were calculated for individuals of 

 each age, sex, reproductive status (im- 

 mature, mature, pregnant), and day 

 of the year by using information on 

 Steller sea lion energetics (basal meta- 

 bolic rates, active metabolic rates, 

 activity budgets, body growth and com- 

 position, digestive efficiency and the 



